Projects::MergeRequests::DiffsController#show is calling Gitaly n+1 times per request

Projects::MergeRequests::DiffsController#show is calling Gitaly n+1 times per request.

This code needs to be refactored to fetch all required information in a single call.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13996/diffs#489fd0c4508eeba85744639e1064bce721860d48_14_13